Middleware in Go

In this lesson, explore middleware in go through one small Go program, and connect its output to the language rules behind it.

Middleware wraps an http.Handler to apply reusable behavior such as logging, authentication, rate limits or request IDs. It runs before and sometimes after calling the next handler.

Order matters: authentication should protect the handler that accesses private data, and response-writing middleware must coordinate when headers are sent.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"net/http"
	"net/http/httptest"
)

func withHeader(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("X-Lesson", "Go")
		next.ServeHTTP(w, r)
	})
}
func main() {
	base := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { fmt.Fprint(w, "ok") })
	rec := httptest.NewRecorder()
	withHeader(base).ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
	fmt.Println(rec.Header().Get("X-Lesson"), rec.Body.String())
}

Output

Go ok

withHeader changes the response header before delegating to the wrapped handler, which writes ok.

• Middleware cannot safely change headers after the downstream handler commits the response.
• Avoid logging secrets from credentials or request bodies.

Add a middleware that rejects requests without a chosen header before reaching the base handler.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Middleware in Go in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.